Developing new therapies to treat advanced prostate cancer

Abstract

Prostate cancer (PCa) is one of the most diagnosed tumours in Australia, pertaining to more than 30% of all cancer detection (AIHW 2020). Most of these diagnoses are correlated to men with early, localised stages of the disease that are easily treated with radical proctectomy surgery and/or radiation. One third of these patients will relapse with their disease progressing to the either the recurrent localised stage or the metastatic stage. These advanced cases will have to undergo Androgen Deprivation Therapy (ADT), as hormone dependence is a key driver of prostate tumorigenesis. ADT will help increase patient survival by halting cancer progression, while minimising symptoms. Although this type of therapy can initially alleviate tumour burden, patients will inevitably be subjected to cancer recurrence where their tumours continue to progress despite castrate levels of androgens. An observed second-order effect of ADT is that it creates a therapeutic selection pressure within PCa, leading to the proliferation and establishment of cells that are resistant to androgen ablation. These hormone-refractory cells lead to the development of Castration Resistant Prostate Cancer (CRPC), a stage of the disease that ultimately leads to death. Despite the development of second-generation ADT or chemotherapeutic agents, no effective therapies have been developed yet to combat this lethal stage of the disease. Therefore, there is a need for therapies that can target pathways and mechanisms that are not related to androgen signalling. MYC overexpression is one of the most frequently characterised genomic aberrations within men with CRPC. MYC mRNA overexpression is found in 30% of men with localised disease and in 85% of men with CRPC. Although MYC seems like a prime therapeutic target to combat PCa, previous attempts at developing MYC inhibitors have failed due to major off-target effects making these therapies dangerous. The overexpression of MYC is tightly correlated with increased ribosome biogenesis because MYC-driven tumours have an inherent need for protein synthesis to sustain infinite cell proliferation. The rate limiting step of ribosome biogenesis is the transcription of a ribosome RNA precursor by RNA polymerase I within the nucleolus. MYC orchestrates the over-transcription of this precursor by tightly interacting with Pol I associated components. PIM kinases and the PI3K pathways also tightly interact with MYC in order to increase its stability and prevent its degradation. It can be inferred from this that MYC-driven tumours are highly proliferative, lack any type of cell cycle regulation, and are constitutively producing proteins to sustain their uncontrolled growth. Administration of CX-5461, an RNA Pol I inhibitor, was followed by potent anti-proliferative activity and cell cycle arrest within various in vitro and in vivo models of prostate cancer. Strikingly, the combination of Pol I inhibition along with CX-6258, a PIM kinase inhibitor, was characterised by potent reduction in invasive lesions within two mouse models of prostate cancer. Although both drugs combined seemed to have potential additive effect, little is understood about the mechanism of action of these two inhibitors together. Therefore, one of the main aims of this thesis is to understand the efficacy of this combination. Both drugs will be tested on different subtypes of human prostate cancer in the explant settings in order to correlate patient-drug response with specific genomic or pathological profiles. A further goal for this project will be to utilize mass spectrometry to characterise the main phosphorylation events that occur as a result of the administration of these inhibitors, both alone and in combination. This will further the understanding of the mechanistic action of both drugs combined. Due to the limited efficacy of CX-5461 as a single agent, PMR-116, a novel Pol I inhibitor, will also be tested within different models of prostate cancer to evaluate as a potential anti-cancer drug. The final aim of this thesis will be to combine CX-5461 with targeted 177Lu-PSMA-617 therapy, as the latter has recently been shown to potentiate radiation-induced DNA damage. Through the various aims of this thesis, we hope to find and characterise the efficacy of RNA polymerase I therapies on prostate cancer both as single agents or combined therapies.